The present invention relates to hydrophones, particularly to an optical fiber hydrophone, and more particularly to a miniature Fabry-Perot fiber optic hydrophone.
It is well known that when an acoustic wave propagates in a medium, the periodic variations in pressure cause corresponding periodic variations in the optical index of the medium. Thus, if a light beam is directed through a straight optic fiber and the optic fiber is subjected to an acoustic wave, the index of refraction of the optic fiber will be changed. As the index of refraction of the optic fiber changes, the phase of the light beam traversing the optic fiber will change. Thus, as the acoustic waves change the index of refraction of the optic fiber, the light passing through the optic fiber changes its phase. This phase change is proportional to the change in the index of refraction of the optic fiber, which is proportional to the pressure variation in the acoustic wave. Therefore, the phase change in the light is a measure of the acoustic pressure field incident on the optic fiber.
Hydrophones utilizing fiber optics have been developed wherein a light passing through an optical fiber is modulated by an acoustic-optic interaction. Such prior art hydrophones are exemplified by U.S. Pat. No. 4,193,130 issued Mar. 11, 1980 to A. M. Young et al.; No. 4,519,252 issued May 28, 1985 to D. H. McMahon; and No. 4,951,271 issued Aug. 21, 1990 to S. L. Garrett et al. In addition, the literature reports an acoustic sensor based on a Fabry-Perot interferometer, with such an interferometer on the end of an optic fiber using the fiber tip as one mirror and a sound receiving diaphragm as the second mirror. See M. Matsumoto et al., Tech. Digest OFS '86, p. 43-46, 1986; and R. Ohba et al., J. Phys. E: Sci. Instrum. 20, 1380-2, 1987.
It is thus seen that the advantages of fiber optic sensors in hydrophone applications are well known. They offer excellent sensitivity, resistance to electromagnetic interference, and all active electronics are kept out of the water. One aspect where they are not clearly superior to conventional piezoelectric transducers is their size. For some applications size is critical and existing designs are too large. Thus, there exist a need in the field of fiber optic hydrophones for a very small hydrophone to satisfy certain applications. This need is satisfied by the present invention in that it provides a very small fiber optic hydrophone based on a Fabry-Perot interferometer.